Distributed hybrid quantum computing 分散型ハイブリッド量子情報処理

博士論文

There are numerous proposals for the physical realization of a quantum<br />computer. However, distributed approaches, making use both of flying and<br />stationary qubits, seem to constitute the most promising route towards a<br />truly scalable device. Such systems guarantee extendibility, they incorpo-<br />rate the interface with communication applications and relax the physical<br />realization of the device, allowing for defect tolerance. Flying qubits are<br />included in the more general concept of a quantum bus, a mediating sys-<br />tem which can be of higher dimension. Such a quantum bus can be used<br />in the straightforward preparation of a standard multi-qubit resource en-<br />abling measurement based quantum computation, the cluster state. This<br />constitutes the framework for the results presented in this thesis.<br />　We begin by investigating the effects of dissipation in the continuous<br />variable bus scheme known as the qubus scheme. By considering loss in the<br />bus as it mediates interactions between the stationary qubits, we obtain an-<br/>alytical results for the effective action of the induced quantum gate. We find<br />that a particular two-qubit gate operates with high fidelity in the presence<br />of moderate loss and give a simple iteration scheme to simplify the effects<br />of loss on the qubits. We then attempt to reduce these effects by preparing<br />the bus in more elaborate state, however no improvements are observed.<br />　We then apply the qubus scheme to the probabilistic generation of cluster<br />states and develop an entangling gate working with high success probability.<br />This allows us to produce cluster states far more efficiently than other pro-<br />posals.　Investigating new methods to analyze the performance of different<br />generation strategies constitutes the second part of this set of results. We<br />begin by making the large flow approximation, used in queuing theory, to<br />obtain the optimal strategy in a regime with large resources. After what<br />we take the other more familiar limit of single cluster growth and introduce<br />absorbing Markov chains as a key mathematical tool.<br />　Finally we look at the transmission of composite quantum systems via a<br />single higher dimensional bus. We provide generalized protocols and inter-<br />actions guaranteeing a full transfer of the information from one composite<br />system to another. These protocols can also serve information process-<br />ing tasks, as useful logical operations can be applied to the data as it is<br />transfered. We notice lastly that the qubus scheme constitutes a potential<br />physical realization.

application/pdf

総研大甲第1201号